Radioisotope heat source with overheat protection



pril i6, 1968 K. E. BUCK 3,377,993

RADIOISOTOPE HEAT SOURCE WITH OVERHET PROTECTION Fled June 28, 1966 26%NVENTOR.

KEITH E. BUCK 53% y 5-4 BYK y Qyf@ /ff-.Mf f77/M ATTORNEY United StatesPatent() 3,377,993 RADIOISOTOPE HEAT SOURCE WITH GVERHEAT PROTECTIGNKeith E. Buck, Alamo, Calif., assigner, by mesne assignments, to theUnited States of America as represented by the United States AtomicEnergy Commission Filed June 28, 1966, Ser. No. 562,429 Claims. (Cl.122-32) This invention relates, in general, to radioisotope heatsources, and, more particularly, to a radioisotope heat source providedwith emergency overheazing pro.ection.

Certain radioisotopes of high specific activity which produce largeamounts of heat from relatively compact fuel forms lare being developedin ever increasing numbers for providing energy in underwater,atmospheric and space applications. Such sources generally utilizephysically stable solid forms, eg., alloys, solid dispersions, chemicalcompounds, ceramics and the like, containing a radioisotope encapsulatedto provide what is termed a radioisotope fuel form or capsule. The fuelforms or capsules are usually disposed in a heat-conductive, Le., heavymetal nuclear radiation shield body to provide a heat source from whichheat is extracted by means of heat transfer agents for use exteriorly,or the heat is applied as by conduction to thermoelectric generatorelements and similar devices for use in proximity to the heat source.

T o prevent heat loss, insulation evacuated spacing with reflectivesurfaces and the like are provided between the heavy metal shield bodyand an exterior housing which is usually in Contact wi.h an exteriorheat sink environment generally at ambient temperature. With such anarrangement, any failure of the heat transfer circuit to provide anample coolant supply or interruption of the heat conduction patheffectively stops the escape or extraction of heat from the source sothat the temperature of the heat source rises in an uncontrolledfashion. Serious damage to the point of melt-down and dispersal of theradioiso tope can occur.

The present invention provides an arrangement in which a sensor elementin thermal contact with the shield body senses the attainment of apredetermined maxiurn safe temperature level. The sensor element thenactuates or otherwise responds to release a heat transfer medium intothe evacuated space between the heat source and outer pressure orcontainment vessel to provide for convective and/or conductive heattransfer between the source and exterior vessel heat sink. The usualinsulating effectiveness of the thermal radiation shields and/orreflective insulating surfaces in said evacuated space is eliminated andthe thermal output of the heat source is transported to the exteriorvessel heat sink to be disposed of harmlessly.

Accordingly, objects of the invention are to provide a radioactiveisotope heat source with improved safety features; to provide a compactoverheat protection system for a radioactive isotope heat source; and toprovide a radioactive heat source protection system actuated by thermalcontact with the shield portion of such a source.

Other objects and advantageous features of the invention will beapparent from the following description and accompanying drawing, inwhich similar reference numerals refer to similar components, of whichdrawings:

FIGURE 1 is a vertical cross sectional schematic view of a radioactiveheat source including a protective system in accordance with theinvention;

FIGURE 2 is a detailed vertical sectional view of a preferred embodimentof a radioactive heat source of the general type shown in FIGURE 1;

3,377,993 Patented Apr. 16, 1968 ice FIGURE 3 is a partial sectionalview of the primary circuit for introducing operating coolant into theheat source of FIGURE 2;

FIGURE 4 is a partial sectional view of the secondary circuit forintroducing operating coolant into the heat source of FIGURE 2; and

FIGURE 5 is an enlarged inset portion of FIGURE 2 showing the actuatingmeans of the protective system.

The overheat protective system of the invention is adaptable for usewith radioactive isotope heat sources corresponding to the generalizedembodiment illustrated in FIGURE l of the drawing. In brief, elongatedgenerally cylindrical fuel capsules 11 comprising a solid stablecomposition including a radioactive isotope enclosed in a cladding aredisposed in tubular channels provided in the central portion of agenerally spherical heavy metal shield 12. For fabrication purposes, theheavy metal shield 12, preferably tungsten, although depleted uraniummay be used, is made in sections as shown by transverse joint lines 13,13', and with a plug section 14 to provide access. To provide optimumthermal conductivity from capsules 11 to shield 12, a heat transfermedium, i.e., helium gas, liquid metal such as NaK (sodium-potassiumeutectic) or the like (not shown) may be disposed to fill the gapsbetween the capsules 11 and shield 12.

A preferred fuel form for use in the capsules 11 is high specificactivity cobalt60cobalt59 mixtures formed in disks and nickel plated,with a plurality of disks encapsulated as in tantalum liner. However, avariety of other radioisotope heat source means such as Sras a titanate;cesium 137 as polyglass, ceramic or other form; Ce-l44 as an oxide, asthe more economical fuels; or any others such as Pm-l47, lio-210,Pu-238, Cnr-242 and Cm-244 in stable solid forms, could also be used,dependent on mission requirements. Nickel base alloys such as HastelloyCand Hastelloy-ZS can be used effectively to further clad the tantalumencapsulated Co-60 fuels and some others, which noble metals such Iasplatinum may be required, eg., for primary cladding of cesium polyglass(borosilicate compositions) and other reactive materials.

For a long duration mission, to produce 3 'kilowatts electrical (3 kwe.)continuously for 10,000 hours at a feasible 14.2% efciency, fuelactivity requirements of various radioisotopes in mega curies aretabulated as follows:

For various reasons, Co-60 is preferred, e.g., high specie activitiesavailable (over 200 euries/gram); high power density of 27 w./cc. (at200 curies/gm.); and halflife of 5.24 yrs., suiciently long to assuresustained power output.

Shield 12 and capsules 11 comprise the primary heat generating means ofthe heat source with heat being delivered by conduction to the exteriorshell surface 16 of shield 12. Heat from such a shell 16, which may takeforms other than spherical, is generally transported in normal operationthrough conduction path, as through thermoelectric elements (not shown)or by some other means such as heat transfer medium means shown inFIGURE 1 and comprising a heat load. More specifically, a first tubularconduit 17 having inlet and outlet ends 1S and 19, respectively,arelwound spirally in bifilar fashion with a second conduit 21 havinginlet and outlet ends 22 and 23, respectively, about shield 12. End 22of conduit 17 may be used for introducing a liquid heat transfer agent26, eg., Dowtherm-A, to be heated and emerge as vapor 27 from conduitend 23 for use exteriorly of the heat source. Auxiliary coolant, e.g.,Dowtherm-A, liquid 26' in a pressurized condition can similarly becirculated through conduit 21 to remove excess heat not expended inequipment (not shown) connected to conduit 21, e.g., in standbyoperating condition, normal emergency or the like, for disposal as iiuidheat exchange medium 27 lby means of heat exchanger or the like (notshown). A closed pressure vessel shell 28, e.g., of nickelbase alloy, isdisposed closely about spiral wound conduits 17 and 21 and a heattransfer medium such as NaK-29 is provided to fill the voids betweenshell 2S and tungsten shield surface 16. The exterior surface 31 ofshell 28 may be polished and other insulating means provided asdiscussed hereinafter to minimize radiation heat loss from surface 31.The arrangement of cornponents enclosed within pressure vessel shell 28may be considered to be a complete radioactive isotope heat sourceassembly 32 adapted to provide a heat transfer medium at elevatedtemperature for driving a turbine, piston engine or other mechanical orelectrical power generating means or other appropriate device exteriorthereto (not shown). The assembly 32 may also be adapted for variablepower output by dividing the heat load supplied by conduits 17 and 21and/or other means as deemed appropriate. For use submerged in ahydrospace environment (ocean, lake, etc.), the heat source assembly 32may be arranged within the lower end of a double spherically endedcylindrical pressure hull 33, eg., of Hastelloy-C or other nickel-basealloy which can serve for heat rejection, i.e., as a heat sink. Morespecically, the generally spherical outer surface 34 of pressure shell28 is disposed in spaced equidistant, i.e., in concentric spaced,relation to the inner surface of lower spherical vessel end 35. Abulkhead 36 is provided in the lower portion of the central cylindricalportion of hull 33 to close ott the lower spherical end 35 defining achamber therein, with conduit ends 18, 19, 22 and 23 being conductedtherethrough in sealed relation. An annular aluminum block 39 canned,eg., in Hastelloy-C, is used to fill the void between bulkhead 37 andupper portions of spherical surface 34 with a concave spherical lowersurface 41 in spaced concentric relation to pressure shell 28 to providefor effective heat conduction from surface 41 to the pressure hull 33for purposes described more fully hereinafter. In a typical operatingdevice, power conversion systems, standby heat load trim means and thelike to be energized by the heat produced or to control assembly 32 arearranged in the cylindrical center and upper spherical end portions (notshown) of hull 33, since same are not particularly relevant to and arenot part of the present invention.

Moreover, the pressure hull 33 and enclosed heat source assembly 32 canbe provided as a heat source unit 50 as shown in FIGURE 2 to facilitateconstruction, transport and the like. Therein, the lower end 35 of thehull is provided with a flanged joint, e.g., a conventional Marman angedjoint 51 in the hernispherical transverse plane of source assembly 32 tofacilitate insertion of assembly 32 therein. (During fabrication,transport, etc., coolant may be circulated through said conduits toavoid overheating in the event convection cooling in air is notadequate.) To provide adequate support, Hastelloy-C spacers S2 (oneshown) are disposed at spaced equisolid angle (e.g., 11) locationsbetween the shield surface 16 and pressure shell 28. To further minimizeradiative heat transfer, at least one dimpled, polished metal heatshield 53 of nickel-based alloy, gold-plated if desired, can be disposedin intermediate position in the void space 54 between the concentricfacing hull and pressure shell surfaces. Proper spacing of heat shield53 is obtained utilizing mullite, or, eg., alumina or other ceramicspacer elements 57 oriented in the same manner as spacers 52 andgenerally in alignment therewith,

and shield 53 is provided with one or more perforations (not shown) toprovide for gas ow from one side to the other. In service, all voidspaces within the lower spherical pressure hull end 35 are evacuated toa low vacuum pressure through ports or the like (not shown) whereuponheat loss between the heat source assembly 32 and pressure hull drops toa very low level, e.g., to less than 2% of the heat output of assembly32.

In practice, conduit 17 ends 18 and 19, respectively, can be brought outof pressure shell 28 and bulkhead 36 utilizing concentric thermal sleeveshield joints 58 and 5), as shown in FIGURE 3. Standby coolant conduit22 can be conveniently provided of concentric arrangement with theinnermost open end of the inner tube ending in proximity to the closedend (not shown) of the outer tube to allow adequate circulation ofcoolant, and with ends 22 and 23 concentrically emerging through athermal sleeve joint 5S and terminating in T-tting 61, permittingattachment to input-output circuits (not shown). The entire heat sourcein the lower end 31 of hull 33 can be made demountable by providing aMarman joint flange 62 on the exterior thereof just above or on a levelwith bulkhead 36.

As discussed above, the heat transfer media in conduits 17 and 21ordinarily conveys the heat output from assembly 32 to external use ordisposal equipment, thereby maintaining the assembly 32 in a safeoperating temperature range. However, if an irremediable interruption ofthe ow or cooling of coolant or heat transfer media supply occurs toseriously curtail removal of heat, insulated assembly 32 can increase intemperature at an uncontrolled rate to the extent that meltdown of theassembly 32 and consequent destruction of valuable apparatus occurs, andthere impends a possibility of escape of hazardous radioactivity frompressure hull 33. Due to the high thermal inertia of the sourcecomponents, the heating does not occur instantaneously, but overheatingcan occur in the relatively short time of several seconds or minutes,dependent on the extent to which normal cooling is impaired. Usually,for high eficiency, the heat source is operated fairly close, e.g., F.to the maximum safe level and accordingly only a restricted increase intemperature can be tolerated.

Emergency cooling under such conditions is provided, in accordance withthe invention, by providing means for releasing a coolant, gaseous oruid in nature, into the evacuated interconnected void spaces,particularly space 54 between the exterior surface 34 of pressure shell28 and concentric surfaces of hull end 35 and annular block 39 on bothsides of the insulating medium, i.e., shield 53, by means actuated by anexcessive temperature increase. Thereupon, convective and conductiveheat transfer between shell 28 and the heat sink provided by hull 33,particularly the lower end of the central cylindrical portion, lower end35 and even by some conduction along bulkhead 36.

More particularly, at least one and preferably a plurality of emergencycoolant storage caviies or vessels 66 shown in FIGURE 1 are disposed orformed in aluminum block 39 or other region providing convenient access.A conduit 67 leads from vessel 66 to open at end 68 into void space 54with a fusible plug 69 being used as the actuator means to close end 68to retain coolant in vessel 66 under normal conditions. Fusible plug 69is preferably disposed in intimate contact with pressure shell 28 toprovide good thermal contact, and is made of a material, c g., metalalloy, having a reliable melting point, original and remelt at apredeterl .ined level, eg., at least about 150 F. above the normaloperating temperature of heat source assembly 32. For a typical sourceusing Dowtherm-A, an exit temperature of the heated vapor 27 may be ofthe order of 700 F. with some portions of source 32 being possibly at ahigher temperature, eg., 750 F. For the construction materials mentionedabove, -a temperature level of the order of 1000" F. would provide asafe level. Several aluminum alloys, listed below, are adequate underthese conditions:

Other fusible plug materials appropriate for different operating levelsmay be selected similarly from standard reference tables, such as thosein the Handbook of Chemistry and Physics (Chemical Rubber Pub. Co.).

Chemically stable heat transfer agents such as mercury metal,Dowtherm-A, etc., pressurized, e.g., with helium in sutlcient volume totill void space 54, may be disposed as emergency coolant in vessels 66.However, there is preferably used an inert gas heat transfer agent suchas helium, argon, neon, nitrogen, CO2 or the like, in an amountsufficient to provide adequate pressure, e.g., at least one atmospherein void space 54 to assure effective heat transfer thereacross.

In FIGURE 2, details of a preferred arrangement are shown in which ametallic bottle 72 (one shown) containing, e.g., helium gas, is slidablydisposed in cylindrical channels 73.supported by an integral tubularcolurn 74 attached at the lower end as by welding to pressure shell 28wherefor there is provided relatively constant spacing of the bottle 72and shell 28 independent of thermal dimensional changes. A conduit tube76 leads downwardly from bottle 72 and includes a lower end portion 77bent to be in intimate contact with and conform to the curvatur-e ofpressure shell 28. Mechanical fastenings, welding or brazng (not shown)can be used to assure contact; however, spring of tube 76 or a springarrangement (not shown) can also be used. The walls of end 77 of tube 76may be provided with perforations (not shown) to facilitate release ofgas, and the lower end 77, including said perforations, is closed withfusible plug material 78 of the type described, along a length of tubein good thermal contact with shell 28. With this arrangement,overheating of shell 28 melts material 78 and releases helium, e.g.,with a pressure of 'above at least 1 atm., into void space 54 to provideemergency cooling as above.

Parameters of a typical heat source (about 24 kw. thermal) suitable fora 3 kwe. (kilowatt electrical), allowing for pumping power, losses andabout 14-15% conversion efficiency, submersible radioisotope powergenerator, are set forth in the following:

TABLE A DEM ONSTRATION FUEL cobalt 0.800 in. ($0.001 in.), dia. by 0.040in. (1L-0.003 in.), thick, plated .all over with nickel 0.0006 in.($00002 in), thick.

Density 8.9 gin/cc.

Specific activity 1 200 curies/ gm.

Power density 1 27.4 w./ cc. (9 watts per disc).

Total activity 1 1.59 106 curies.

1 At the ytime of fueling.

The cobalt fuel is doubly encapsulated in a rtantalnm liner and aHastelloy-C capsule. Helium-filled gaps are provided inside bothcontainers to facilitate leak checking. The fuel is distributed among 19fuel pins or eapsules, each pin containing 143 cobalt discs. The pinsare placed in holes arranged in a tungsten matrix at the center of thetungsten shield. Fueling is accomplished by handling each pin separatelyto minimize requirements for heat dissipation. Dimensions of the capsuleand `array are given in Table B. The axial clearance provides fordifferential thermal expansion of the fuel and shield (the thermalcoefficient for HastelloyC is approximately three times that oftungsten). The cladding and liner are shrinktted at room temperature,and the capsule and hole diameters are equal at nominal operatingtemperature. These close fits will minimize thermal conductionresistance between the fuel and tungsten. The tungsten holes may, ifnecessary, be coated with inert material for a barrier between thetungsten and Hastelloy-C fuel cladding to prevent self-welding.

TABLE B.FUEL CAPSULE AND ARRAY DIMENSIONS FOR THE RISE DEMONSTRATIONUNIT Total fuel volume en. in 54. 56 Fuel VOL/disc, cu. in. 0.0201 FuelVOL/pin (19 pins), cu in 2. 871

No. discs/piu.. 143

60 F. 1,400o F.

Plated fuel diameter, in 0. 801 0.801 Tantalum liner:

Inside diameter 0.808 0.812 Outside diameter 0.848 0.852 Hastelloy-Cclad, in.:

Inside diameter 0. 848 0. 858 Outside diameter 0.948 0.959 Tungsten holediameter, in-. 0. 955 0. 959

The volume of boiler core material, volume fractions, and effectivedensity are listed in Table B. These parameters were calculated usingroom temperature dimensions based on a hexagonal array and assuming thatone-half the tungsten web thickness outside the outer fuel capsulesshould be counted as core material rather than shield material.

TABLE C.-BOILER CORE The 1.17 and 1.33 mev. gamma rays emitted bycohalt-60 must be shielded to prevent radiolysis of the Dowtherm-Aworking fluid and to meet radiation protection criteria. Approximately 3in. of tungsten would reduce the gamma dose rate to a level WhereDowtherm-A radiolysis will be insignificant over a 10,000-hr. operatingperiod; but approximately 7 in. of tungsten are required for biologicalsafety. The Dowtherm-A tubing could, therefore, be located between splitshielding; however, this conguration would increase both shield weightand fabrication problems, and it is expedient, therefore, to locate theDowtherm-A boiling coil outside the biological shield.

Although tungsten is a less etlicient gamma ray shield than uranium perunit weight, tungsten was chosen as the reference shield materialbecause of superior thermal conductivity (roughly three times greaterthan that of uranium). The temperature difference between the boilercore and the Dowtherm-A coil is therefore roughly onethird as great withtungsten as with uranium.

Large gamma ray sources absorb a high percentage of the gamma ray energyproduced in the source. This effect in enhanced in this heat sourcebecause of the relativery large volume fraction of tungsten in the lfuelyregion. Calculations indicate that 74% of Ithe total power is generatedin the fuel region, while 26% is generated in the shield, primarily inthe 2 in. closest to the fuel. Roughly 5% of the thermal power is due tothe low energy beta radiation which is entirely absorbed within thecore.

In the boiler tube 17, 336 lb./hr. of Dowtherm-A is preheated to thesaturation temperature (697 F.), is vaporized, and then superheated 3 F.This is Iaecomplished in a 78-ft. long, 0.625-in. OD, 0.575-in. IDHastelloy-C tube that is wound around the exterior of the shield. Thereare three modes of heat transfer in the tube: forced convection to aliquid in the preheating region; two-phase boiling in the low qualityvaporizing region; and forced convection to a vapor in the superheatingand high quality vaporizing regions. For this analysis, it was assumedthat the transition between two-phase boiling and forced convection tovapor occurs at 70% Dowtherm-A quality.

To prevent pyrolysis of the Dowtherm-A at the tube wall, it is desirableto maintain the lowest possible tube wall temperature. The 78-ft. boilertube length was selected because it provides a reasonable pressure dropof 6% p.s.i. and a maximum tube wall temperature of only 735 F.

TABLE D 60 F. Operating Temp.

Inner Vessel (28):

Inner radius, in 10. G30 10. 091 Outer radius, in 10. 930 10. 993Thickness, in 0. 300 0. 302 Standotls (11):

Area, sq. in 0. 442 0.442 Length, in 0. 750 O. 750 Outer Vessel (33):

Inner radius, in 11. O88 11. 093 Outer' radius, in 11.928 11. 933Thickness, in O. 840 0. 840 Spacers (11):

Aren, sq. in 0.283 0. 288 Length, in 0.200 0. 200 Vacuum SpaceThickness, u1.1 0.158 0.100 Tungsten shield (spherical radius), in 9. 88Shield plug (diameter), in 7.00

1 May be made larger if necessary to accommodate heat sheld 53.

TABLE E Operating Component: temperature, F. Fuel-cladding interface1660 Cladding-shield interface 1620 Dowtherm-A coolant tubing:

In 562 Out 740 Shield-periphery 800 Inner pressure vessel 800 Outerpressure vessel 1 30 to 85 F.

1 Ocean temperature.

The outer pressure hull 33 consists of a 24-in. diameter cylinder havinga length of 88 in. and a Wall thickness of 0.75 in. with hemispheresenclosing each end of the cylinder. Internal rib stilfeners are in theform of standard I-beams, 3 by 2% in. equally spaced along the length ofthe cylinder. The hemispherical ends are 0.84-in. thick and are weldedonto the cylindrical shell to form the lprimary containment vessel.

The inner pressure shell 28 is spherical to conform to the geometry ofthe tungsten shield. Its main purposes are to contain the N aK heattransfer uid, `form one surface of the vacuum insulation gap and toprovide secondary (the outer pressure containment vessel is primary)protection for the fuel, Dowtherm-A coil, shield and NaK against thedestructive effects of the high pressure seawater environment should theouter pressure vessel fail. The tungsten shield, containing the fuel, ispositioned in the center of the inner pressure vessel by Hastelloy-Cstandoffs. These standoffs, which are designed to withstand a 6 g. shockload, maintain clearance between the pressure vessel and tungsten forthe DoWtherm-A coils and provide radial support for the inner pressurevessel if the outer pressure hull ruptures.

The thickness of the pressure vessel wall is based upon the stress forpressure of 1000 p.s.i.a., but not for buckling. The result is a wallthat will not resist the full force of 1000 p.s.i.a. external pressurewithout the probability of buckling instability; however, Hastelloy-C issutiiciently ductile to collapse upon the tungsten shield withoutrupturing.

The inner pressure vessel is perforated in three places for passage ofthe Dowtherm-A lines from the boiler to the power conversion equipmentand for the standby cooling system lines. The Dowtherm-A lines arejoined to the pressure vessel by thermal sleeves to minimize thetemperature gradient between the tubes and pressure vessel Wall. Anevacuated region is provided to separate the inner pressure vessel, theouter pressure vessel and Athe bulkhead which lies between the boilerand the PCS. The vacuum limits the heat loss to the seawater duringnormal conditions.

The vacuum gap is maintained by ceramic (mullite) spacers between `theinner and outer pressure vessel. The vacuum gap is designed to ensure a0.100-in. gap at operating conditions, and includes provision for0.0l8-in. Icontraction of the outer pressure vessel as a result of the1000 p.s.i.a. external pressure and for 0.047-in. thermal expansion ofthe inner pressure vessel. Penetrations through the vacuum gap by theDowtherm lines are seal-welded at each pressure vessel to assure leaktightness.

The Dowtherm-A lines are sufficiently strong to contain the internalpressure of the Dowtherm-A at operating temperatures. If the primarypressure vessel ruptures, the tubes are expected to resist collapse thatmight result from 1000 p.s.i.a. seawater pressure. Calculations showthat the tubes will resist buckling if the tubing is not oval wherebends are used.

The bulkhead separating the boiler from the power conversion compartmentconsists of a 0.54-in. thick flat plate having radial rib-type stienersto limit the deiiection of the plate and the attached Dowtherm-A linesduring shock loading. Thermal sleeves are also provided for theDowtherm-A lines where they penetrate the bulkhead en route from theboiler to the power conversion unit.

While one fusible plug or link actuator means has been described indetail, it may be noted that a rupture disc can be disposed in contactwith and supported by the fusible plug 78 in the conduit 76 to insureagainst inadvertent leakage of emergency coolant into void space 54.Other types of thermally-actuated devices, eg., thermal expansion,bimetallic, thermostatic disc element controlled or driven puncturemeans, substituted lfor said fusible plug, could be used to rupture adiaphragm or rupture disc used to close conduit 76. Likewise, apuncturing device driven by a fusible plug could be used. Valvesactuated by thermoelectric sensors or the like could also be used;however, bulky components disposed exterior to the spherical end of hull33 are generally not desirable. For use in environments other thanwater, blown air, radiator tins, contact heat exchangers and the likemay be used to dissipate excess emergency heat from hull 33.

Other modifications will be .apparent to those skilled in the art, andit is intended to cover all such together with those specificallydisclosed in the appended claims.

What is claimed is:

1. In a source for supplying heat to a load, the combination comprising:

(a) heat generating means comprising a massive body of heat conductivematerial including a non-interruptable means for continuously generatingheat therein, said body being provided with means for transferring heatgenerated therein to an external heat load;

(b) heat sink means including a surface in spaced relation to exteriorsurface portions of said massive body .and defining therewith a voidspace evacuated to minimize convective and conductive heat transferthereacross under normal operating conditions wherefor overheating ofsaid massive body can occur in interruption of said heat transfer to anexternal load; and

(c) means including a pressurized emergency coolant source, a conduitVleading from said source to communicate with said void space, .andclosure means normally retaining said coolant in said source, saidclosure means arranged in thermal communication with said massive bodyto sense overheating of said body and actuate release of said coolantinto said void space so that the heat generated therein is transferredharmlessly across said void space to said heat sink 2. Apparatus asdefined in claim 1, wherein said massive body is a heavy metal radiationshield and said heat generating means comprises an encapsulatedradioisotope fuel form disposed centrally in said shield.

3. Apparatus as defined in claim 2, wherein said radioisotope fuel formincludes a material selected from the group consisting of Co-60, Sr-90,Cs-137 and Ce-114.

4. Apparatus as defined in claim 3, wherein said massive body of saidheat generating means is a generally spherical tungsten metal shield,said radioisotope fuel form therein is an encapsulated cobalt-60metallic mixture with cobalt-59, and said heat sink comprises a metallichull having spherical surfaces generally concentric to the surfaces ofSaid tungsten shield and spaced therefrom across said void space.

5. Apparatus as defined in claim 4, wherein said means for transferringheat from said heat generating means comprises a boiler pressure vesselshell `spaced inwardly across said void space with respect to saidmetallic hull spherical surfaces, encompassing said tungsten metalshield in concentric spaced relation, said actuation closure means is inthermal communication with said boiler shell, means for introducing andcirculating at least one heat transfer medium into said boiler vessel tobe heated, thenceforth to be conducted exteriorly to deliver the heatedmedium to said external heat load.

6. Apparatus as defined in claim 5, wherein said emergency coolantsource closure means comprises a fusible material disposed to plug aportion of said conduit in thermal communication with said boiler vesselshell, said material having a melting point at a predetermined safelevel above the normal operating level of said heat generating means.

7. Apparatus as defined in claim 6, wherein means for introducing andcirculating a heat transfer medium into said shell comprises acontinuous conduit spirally wound in the space between said boiler shelland shield, and a heat transfer agent is disposed in the interstitialspaces within said boiler vessel shell.

8. Apparatus yas defined in claim 7, wherein a dimpled, poli-shedmetallic radiant heat shield is disposed in said void space intermediatebetween said concentric shell and heat sink hull surfaces.

9. Apparatus as defined in claim 8, wherein said emergency coolant is aninert gas heat transfer agent.

10. Apparatus as defined in claim 9, wherein the heat transfer agentdisposed in the interstitial spaces within said boiler shell is a liquidmetal, and the heat transfer agent circulated through said conduit is ahigh temperature, radiation resistant organic heat transfer medium.

References Cited UNITED STATES PATENTS ll/l96l Fraas et al. 176

OTHER REFERENCES CHARLES J. MYI-IRE, Primary Examiner.

1. IN A SOURCE FOR SUPPLYING HEAT TO A LOAD, THE COMBINATION COMPRISING: (A) HEAT GENERATING MEANS COMPRISING A MASSIVE BODY OF HEAT CONDUCTIVE MATERIAL INCLUDING A NON-INTERRUPTABLE MEANS FOR CONTINUOUSLY GENERATING HEAT THEREIN, SAID BODY BEING PROVIDED WITH MEANS FOR TRANSFERRING HEAT GENERATED THEREIN TO AN EXTERNAL HEAT LOAD; (B) HEAT SINK MEANS INCLUDING A SURFACE IN SPACED RELATION TO EXTERIOR SURFACE PORTIONS OF SAID MASSIVE BODY AND DEFINING THEREWITH A VOID SPACE EVACUATED TO MINIMIZE CONVECTIVE AND CONDUCTIVE HEAT TRANSFER THEREACROSS UNDER NORMAL OPERATING CONDITIONS WHEREFOR OVERHEATING OF SAID MASSIVE BODY CAN OCCUR IN INTERRUPTION OF SAID HEAT TRANSFER TO AN EXTERNAL LOAD; AND (C) MEANS INCLUDING A PRESSURIZED EMERGENCY COOLANT SOURCE, A CONDUIT LEADING FROM SAID SOURCE TO COMMUNICATE WITH SAID VOID SPACE, AND CLOSURE MEANS NORMALLY RETAINING SAID COOLANT IN SAID SOURCE, SAID CLOSURE MEANS ARRANGED IN THERMAL COMMUNICATION WITH SAID MASSIVE BODY TO SENSE OVERHEATING OF SAID BODY AND ACTUATE RELEASE OF SAID COOLANT INTO SAID VOID SPACE SO THAT THE HEAT GENERATED THEREIN IS TRANSFERRED HARMLESSLY ACROSS SAID VOID SPACE TO SAID HEAT SINK. 